1. Field of the Invention
The invention relates to a method for temperature management in an exhaust gas posttreatment system of an internal combustion engine, having an exhaust gas duct in which an SCR catalytic converter is provided in the flow direction of the exhaust gas, and a reductant generating system (RGS) has both an NOx and CO/H2 generating unit and a combined NOx reservoir/ammonia generating unit (AGC unit) in the standard gas course of the reductant generating system, and for reducing nitrogen oxides, ammonia is supplied as reductant by the reductant generating system upstream of the SCR catalytic converter, and the NOx and CO/H2 generating unit is at least intermittently supplied via a fuel supply and an air supply with starting materials for generating the ammonia. The invention also relates to a corresponding apparatus.
2. Description of the Prior Art
For reducing nitrogen oxides in the exhaust gas of engines operated with a lean fuel mixture, NOx storage catalytic converters, also called NOx storage/reduction catalytic converters or NSCs, can be used. These NOx storage catalytic converters function discontinuously in a mode that comprises two phases: In the first, longer phase or so-called lean phase (Lambda>1), the nitrogen oxides from the engine that are contained in the exhaust gas are stored. In the second, shorter phase, the so-called rich phase (Lambda<1), the stored nitrogen oxides are regenerated by means of rich exhaust gas generated inside the engine. In the regeneration, in the conventional mode of operation of an NSC, only nitrogen (N2), water (H2O), and carbon dioxide (CO2) are produced from the stored nitrogen oxides.
It is fundamentally known that under unfavorable regeneration conditions, such as a very long regeneration and/or low Lambda value (λ≈0.8), a more likely small proportion of the stored NOx can be converted to ammonia (NH3). In that case, however, the NH3 formation is an unwanted, parasitic effect.
In connection with future specifications in terms of nitrogen oxide emissions from motor vehicles, suitable exhaust gas posttreatment is necessary. Selective catalytic reduction (SCR) can be used to reduce NOx emissions (removal of nitric oxides) in internal combustion engines, especially diesel engines, with intermittently predominantly lean or in other words oxygen-rich exhaust gas. In this process, a defined quantity of a selective-action reductant is added to the exhaust gas. The reductant may for instance be in the form of ammonia, which is metered in directly in gaseous form, or is also obtained from a precursor substance in the form of urea or from a urea-water solution (UWS). Such UWS-SCR systems were used first in utility vehicles.
In German Patent Disclosure DE 10139142 A1, an exhaust gas cleaning system in an internal combustion engine is described, in which to reduce NOx emissions, an SCR catalytic converter is used, which reduces the nitrogen oxides that are in the exhaust gas to nitrogen using ammonia as the reagent. The ammonia is obtained from the urea-water solution (UWS) in a hydrolytic catalytic converter located upstream of the SCR catalytic converter. The hydrolytic catalytic converter converts the urea, contained in the UWS, into ammonia and carbon dioxide. In a second step, the ammonia reduces the nitrogen oxides to nitrogen, creating water as a byproduct. The precise sequence has been extensively described in the professional literature (see Weissweller in CIT (72), pages 441-449, 2000). The UWS is furnished in a reagent tank.
It is disadvantageous in this method that UWS is consumed in the operation of the internal combustion engine. Its consumption is approximately 4% of the fuel consumption. The supply of urea-water solution would have to be assured over a suitably large area, for instance at service stations. Another disadvantage of the method is the necessary operating temperature range. The hydrolytic reaction of the urea-water solution does not occur quantitatively at the hydrolytic catalytic converter, releasing ammonia, until temperatures of more than 200° C. In diesel engines, for instance, these exhaust gas temperatures are not reached until after a relatively long period of operation. At temperatures below 200° C., deposits can cause clogging of the metering unit, which at the very least is a hindrance to delivering the urea-water solution to the exhaust gas system. Adding the urea-water solution at temperatures below 200° C. can also, because of polymerization, inhibit the necessary catalytic properties of the hydrolytic catalytic converter of the SCR catalytic converter.
German Patent DE 199 22 961 C2 describes an exhaust gas cleaning system for cleaning the exhaust gas of a combustion source, in particular a motor vehicle internal combustion engine, of at least the nitrogen oxides contained in it, using an ammonia generating catalytic converter for generating ammonia, using ingredients of at least some of the exhaust gas emitted by the combustion source during ammonia-generating phases of operation, and also using a nitrogen oxide reducing catalytic converter downstream of the ammonia generating catalytic converter, for reducing nitrogen oxides contained in the emitted exhaust gas from the combustion source, using the generated ammonia as a reductant. In this system, a nitrogen oxide generating unit that is external to the combustion source is provided for enriching the exhaust gas, supplied to the ammonia generating catalytic converter, with nitrogen oxide generated by it during the ammonia generating phases of operation. A plasma generator is proposed for instance as the nitrogen oxide generating unit, for plasma technology oxidation of nitrogen, contained in a delivered gas stream, to nitrogen oxide. The hydrogen required for generating the ammonia is generated during the ammonia generated phases of operation by operating the combustion source with a rich or in other words fuel-rich air ratio.
A disadvantage of this method is the relatively high fuel consumption during the requisite rich phases of operation. Furnishing the nitrogen oxide to the engine externally also dictates high energy usage, especially since nitrogen oxide has to be produced in high concentration during the ammonia generating phases, which have to be as short as possible, and the remaining residual oxygen for generating ammonia has to be removed in a way that is expensive in terms of energy. If the hydrogen is generated via a POx catalytic converter by means of partial oxidation reforming (POx), then a further disadvantage the heretofore poor dynamics of generating hydrogen results.
A method for generating a hydrogen-rich gas mixture using plasma chemistry is described in International Patent Disclosure WO 01/14702 A1. In it, a rich fuel-air mixture is treated in an electric arc, preferably under POx conditions.
To avoid having to carry an additional fuel as well, a plasma method for on-board generation of reductants has been proposed in an as yet unpublished document of the present Applicant. In it, the ammonia required for reducing the nitrogen oxides is produced from nontoxic substances as needed in the vehicle and then is delivered to the SCR process. An acceptable solution in terms of fuel consumption is offered by a discontinuous method for ammonia generation, of the kind also proposed in the same document. This method will hereinafter be called the RGS method (Reductant Generating System), or reducing agent generating system.
One important component of an RGS unit is a catalytic converter, which while it does operate on the discontinuous fundamental principle of an NOx storage catalytic converter (NSC), is nevertheless operated such that the nitrogen oxides, stored in the lean phase, are converted in a targeted way in the rich reduction phases into ammonia, rather than into nitrogen oxide. The nitrogen oxides are produced under lean conditions, for instance from air, in a nitrogen oxide generating unit that is combined with a hydrogen/carbon monoxide generating unit to make an NOx and CO/H2 generating unit. This CO/H2 generating unit is also called a reductant generating unit. The gas mixture leaving this unit in the rich phases predominantly comprises H2, CO, and N2, and is also called reformate gas. The ammonia generated periodically (that is, cyclically) in this way is metered to the exhaust gas train of the engine and is converted with NOx from the engine to N2 in the downstream SCR catalytic converter. This kind of NOx storage catalytic converter operated with maximum NH3 is also called a combined NOx reservoir/ammonia generating unit (AGC stands for “ammonia generating catalyst”).
The operating conditions of the combined NOx reservoir/ammonia generating unit for targeted generation of ammonia outside the exhaust gas train are extremely different from those of a conventional NSC in the full exhaust gas stream. Essentially, the differences are these:    an approximately 10 to 20 times higher concentration of NOx (up to 1%) and of H2/CO (totaling up to 40%);    typically markedly higher global NOx load densities of the NOx storage catalytic converter (up to 2 g NO2 per liter of AGC volume), and associated with this,    extremely exothermic heat tonalities over the length of the catalytic converter in the combined NOx reservoir/ammonia generating unit, with positive temperature gradients ΔT of over 100° C.
The ammonia yield in the combined NOx reservoir/ammonia generating unit depends on the temperature management at the combined NOx reservoir/ammonia generating unit, or the temperature profile over the length of the combined NOx reservoir/ammonia generating unit; on the duration of the rich phase; on the concentration of reductant agent; and on the catalytic converter formulation; the catalytic converter formulation generates a characteristic temperature profile and can affect the ammonia selectivity. It has been demonstrated that the formation selectivity is high enough only within a narrow temperature window.
It is therefore the object of the invention to furnish a method with which a high ammonia formation rate can be assured even under changing loading faces. It is also the object of the invention to furnish a suitable apparatus for carrying out the method.